1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a zoom lens suitable for use in an image pickup apparatus such as a digital camera, a video camera, a TV camera, and a monitoring camera.
2. Description of the Related Art
In recent years, an image pickup apparatus such as a video camera and a digital camera using a solid-state image sensor has been miniaturized while maintaining a high functionality. In addition, as the image pickup apparatus is miniaturized with a high functionality, an optical system used therein is required to be a miniaturized zoom lens having excellent optical performance such as a wide angle of view, a high zoom ratio, and a high aperture ratio. Furthermore, there is a demand for a retractable zoom lens in which a projected length of the lens from the camera body is reduced by retracting each lens unit up to a distance different from that of the imaging state when it is not used in order to obtain a compact camera and a high zoom ratio of the zoom lens.
As a zoom lens with a miniaturized entire zoom lens, a wide angle of view and a high zoom ratio, there is known a negative lead type zoom lens in which a lens unit of a negative refractive power is closest to an object side. As the negative lead type zoom lens, there is known a three-unit zoom lens including a first lens unit of a negative refractive power, a second lens unit of a positive refractive power, and a third lens unit of a positive refractive power in order from an object side to an image side (refer to Japanese Patent Application Laid-open No. 2007-108531 and U.S. Pat. No. 6,124,984).
In order to obtain miniaturization of the entire zoom lens and maintain the high zoom ratio in the negative lead type three-unit zoom lens, it is effective to increase the refractive powers of each lens unit of the zoom lens. In addition, in order to obtain a compact image pickup apparatus, it is effective to use the retractable zoom lens in which a projected length of the lens from the camera body is reduced by retracting each lens unit up to a distance different from that of the imaging state when it is not used. However, for example, if the refractive powers of each lens unit are simply strengthened to obtain a wide angle of view and a high zoom ratio and miniaturize the entire zoom lens, a variation of aberration caused by zooming increases so that it is difficult to obtain excellent optical performance across the entire zoom range.
Moreover, if the movement amounts of each lens unit for zooming and focusing increase, the entire lens length increases accordingly so that a complicated lens barrel configuration is necessary to obtain a desired retraction length. In general, since the movement amount of each lens unit strongly relates to the zoom ratio, such a tendency becomes significant as the zoom ratio increases. In addition, in order to obtain a high aperture ratio, a lens unit having an aperture stop becomes large-sized, and a diameter of a marginal ray increases. As a result, a significant number of unnecessary beams are incident to the image plane so that optical performance is degraded. For this reason, it is necessary to provide a countermeasure to sufficiently correct aberration such as increasing the number of lenses.
If the number of lenses of each lens unit increases, the entire lens length for retraction increases so that it is difficult to miniaturize a camera. For this reason, in the negative lead type three-unit zoom lens, it is important to appropriately set configurations of each lens unit in order to obtain a desired retraction length while the entire zoom lens is miniaturized with a wide angle of view and a high zoom ratio. For example, if a wide angle of view and a high zoom ratio are obtained without appropriately setting a configuration or a refractive power of the second lens unit, it is difficult to obtain excellent optical performance. In addition, the entire zoom lens becomes large-sized.
Although Japanese Patent Application Laid-open No. 2007-108531 described above provides a zoom lens having a focal length of about 36 mm and a zoom ratio of about 3.0 in the case of a 35 mm silver-halide film camera, the capturing angle of view is not a wide angle of view, and the zoom ratio is not sufficient. In addition, although U.S. Pat. No. 6,124,984 proposes a zoom lens having an F-number of 2.7, the image-side lens unit is large-sized due to an aperture stop, an aperture ratio is not negligible and brightness is not sufficient.